1. Field of the Invention
The present invention relates to a method of processing a superconducting thin film such as forming a fine groove on a superconducting thin film, and a method of producing a Josephson junction device by utilizing such a processing method.
2. Description of the Related Art
In a superconductive device such as a superconducting transistor and a superconducting quantum interference device, a Josephson junction such as SNS (superconductor. normal conductor. superconductor) junction and SIS (superconductor.insulator.superconductor) junction is adopted. A Josephson junction is easily effected by deposited SNS layers in series. However, when thin films are deposited in this way, it is difficult to attach electrodes to each thin film. As a countermeasure, each thin film is separated into a plurality of parts by forming a minute groove of the like on the thin film, and an electrode is attached to each part. Since this is a planar structure in which two superconducting thin films are arranged in parallel in the same plane, attachment of electrodes or the like is greatly facilitated.
As a method of forming a fine groove or the like on a thin film of an oxide super conductor such as Bi-Ca-Sr-Cu-O, an etching method such as ion beam sputter etching and laser assisted etching are known. As an example of ion beam sputter etching, a method of processing a thin film by focused ion beams (FIB) is shown in FIG. 13. This is a method of irradiating a superconducting film 12 on a MgO substrate 11 with ion beams 13 at an accelerating energy of several ten to several hundred KeV and focussed into about 1 to 0.1 .mu.m so as to form a groove 15 by sputtering.
Another example of ion beam sputter etching is shown in FIG. 14(a). This is a method of forming a desired pattern on the superconducting thin film 12 with a resist 16 which is deposited thereon, irradiating the entire part of the thin film 12 with ion beams 13 of Ar or the like so as to form the grooves 15 by sputtering the parts which are not covered with the resist 16, as shown in FIG. 14(b), and thereafter removing the resist 16, as shown in FIG. 14(c). (H. Tsuge et al., Jpn. J. Appl. Phys. Vol. 27, No. 11, November, 1988, pp. L2237 to L2239).
Laser assisted etching is a method of immersing a superconductor (or superconducting thin film) 6 in an aqueous solution 7 of KOH, and irradiating the superconductor 6 with laser beams 8 so as to rise the temperature of the irradiated part and dissolve the irradiated part of the superconductor 6 in the aqueous solution 7, as shown in FIG. 15. (B. W. Hussey and A. Gupta, Appl. Phys. Lett. 54 (13), 27 March 1989, pp. 1272 to 1274).
The above-described conventional methods, however, have the following problems.
Firstly, in ion beam sputter etching, since the constitutional substances of the superconducting thin film are sputtered out, a damaged layer is formed on the side surface of the groove obtained by sputtering. Due to the existence of the damaged layer, the effective width of the gap in the superconducting thin film is made larger than the width of the groove. Although the crystallinity of the damaged layer is recovered to a certain extent by appropriate annealing, since the shape of the groove changes, particularly fine grooves may inconveniently be brought into contact with each other.
Secondly, when a particularly narrow groove is formed, the elements of the superconducting thin film which are to be etched are difficult to remove and adhere again to the side surface of the processed groove. Therefore, the width of the processable groove is limited, and since the layer of adhered elements is not a superconductive layer, the effective gap is made larger than the apparent width of the groove.
Thirdly, in ion beam sputter etching, control of the dosage for the purpose of controlling the etching depth is very delicate and difficult. That is, if the dosage is too large, the substrate is also etched, while too small a dosage leads to the existence of a residual superconducting thin film.
On the other hand, in laser assisted etching, since the spot diameter of the laser beam with which the superconductor is irradiated is limited by diffraction, and the effect of local rise of the temperature is utilized, it is very difficult to form a very fine groove such as a groove of about 0.1 .mu.m wide.